[jMacKenzie]

Hello,

I have been asked to use (1S,2R)-1-bromo-1-sec-butyl-2-methylcyclohexane in an E2 reaction.  My question is to which is the major product.  I want to say it is structure B, as it has the more substituted alkene.  But we have been discussing E2 reactions with cyclohexanes with regards to the LG needing to be in the axial position and the beta proton anti to it, even is it produces a non-Zaitsev product, so I feel that A maybe the better answer.  Any insight would be greatly appreciated (it is possible that I drew the reactant incorrectly).

Thanks!

[phth]

syn and anti periplanar transition state are possible.  This may be a trick quetsion in reality a double bond will move inside the ring instead of moving to a more saturated double bond.   Kindergarten organic chemistry would say structure B, but in reality I think if you did a scifinder search you may find that A is the major component or it is somehwere near 1:1 mixture.

[jMacKenzie]

Thanks for answering but would you elaborate (this forum states that it is for students and as such I am just learning and also don't have scifinder access)? I thought anti periplanar was the only possibility for E2 (that is what I was taught anyway).  Also, why would it would the double bond prefer to move inside the ring versus being more substituted?

Thanks again, all help is greatly appreciated

[orgopete]

I agree with phth on this one in that both products are possible. I also agree this question can be ambiguous. In addition, I have been of the opinion the factors in these types of questions may not be completely understood. You may find different results for NaOEt/EtOH reactions versus KOtBu/toluene reactions. I generally find NaOEt/EtOH examples give the Zaitsev products. I think these reactions have additional SN1 character to them in which the proton lost is from the more substituted (electron rich) carbon. That would lead to product A.

Although this example is not suited to formation of a more kinetic-like product, but I think if 1-chloro-1-methylcyclohexane were treated with KOBu, the major product would be methylenecyclohexane. I reason the greater acidity of a methyl group plays the major role in the product formation. That is different than the example given here, but are generally considered together in E2-eliminations.

[Babcock_Hall]

@OP,

Why do you think that an anti-periplanar arrangement is an advantage?  Would a syn-periplanar arrangement confer some of the same advantage?